Map Object... help

I'm trying to use the "Map Object..." filter under "Filters > Map > Map Object..." and sometimes it's available, but most of the time it's grayed out. I can't figure out under what circumstances I can and cannot use "Map Object...". I thought maybe it depended on file type, but that doesn't seem to make a difference, and I thought maybe the image had to be a square, but that doesn't seem to do the trick either. I'm using the Windows version of GIMP (Windows 7). Does anyone have any suggestions?

In Gimp, an image can bee in either ofr three modes: RGB (full color, the usual mode), grayscale, and indexed (milited number of colors, typically for GIF). Many filters work only on RGB images, and appear grayed out when the image isn't RGB (or when applied to grayscale only things, such as channels and masks).